linux-kernel - Re: [PATCH v2 10/11] mm/vmstat: switch vmstat shepherd to flush per-CPU counters remotely

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Date:   Thu, 2 Mar 2023 18:16:42 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Christoph Lameter <cl@...ux.com>,
        Aaron Tomlin <atomlin@...mlin.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2 10/11] mm/vmstat: switch vmstat shepherd to flush
 per-CPU counters remotely

On Thu, Mar 02, 2023 at 04:01:07PM -0500, Peter Xu wrote:
> On Thu, Feb 09, 2023 at 12:02:00PM -0300, Marcelo Tosatti wrote:
> > +#ifdef CONFIG_HAVE_CMPXCHG_LOCAL
> > +/* Flush counters remotely if CPU uses cmpxchg to update its per-CPU counters */
> > +static void vmstat_shepherd(struct work_struct *w)
> > +{
> > +	int cpu;
> > +
> > +	cpus_read_lock();
> > +	for_each_online_cpu(cpu) {
> > +		cpu_vm_stats_fold(cpu);
> 
> Nitpick: IIUC this line is the only change with CONFIG_HAVE_CMPXCHG_LOCAL
> to replace the queuing.  Would it be cleaner to move the ifdef into
> vmstat_shepherd, then, and keep the common logic?

https://lore.kernel.org/lkml/20221223144150.GA79369@lothringen/

Could have

#ifdef CONFIG_HAVE_CMPXCHG_LOCAL
int cpu_flush_vm_stats(int cpu)
{
	return cpu_vm_stats_fold(cpu);
}
#else
int cpu_flush_vm_stats(int cpu)
{
	struct delayed_work *dw = &per_cpu(vmstat_work, cpu);

	if (!delayed_work_pending(dw) && need_update(cpu))
		queue_delayed_work_on(cpu, mm_percpu_wq, dw, 0);
}
#endif

static void vmstat_shepherd(struct work_struct *w)
{
       int cpu;

       cpus_read_lock();
       for_each_online_cpu(cpu) {
	       cpu_flush_vm_stats(cpu);
               cond_resched();
       }
       cpus_read_unlock();

       schedule_delayed_work(&shepherd,
               round_jiffies_relative(sysctl_stat_interval));
}

This looks really awkward to me. But then, we don't want
schedule_delayed_work if !CONFIG_HAVE_CMPXCHG_LOCAL.
The common part would be the cpus_read_lock and for_each_online_cpu
loop.

So it seems the current separation is quite readable
(unless you have a suggestion).

> > +		cond_resched();
> > +	}
> > +	cpus_read_unlock();
> > +
> > +	schedule_delayed_work(&shepherd,
> > +		round_jiffies_relative(sysctl_stat_interval));
> > +}
> > +#else
> >  static void vmstat_shepherd(struct work_struct *w)
> >  {
> >  	int cpu;
> > @@ -2026,6 +2043,7 @@ static void vmstat_shepherd(struct work_
> >  	schedule_delayed_work(&shepherd,
> >  		round_jiffies_relative(sysctl_stat_interval));
> >  }
> > +#endif
> >  
> >  static void __init start_shepherd_timer(void)
> >  {
> > 
> > 
> > 
> 
> -- 
> Peter Xu
> 
>